Off days, off weeks, heck, I had an off month recently. It's part of the experience.
Might be a sign that you need a more substantial break, especially if - like me - you aim for 9-5 days, 5 days a week, and haven't had any real time away for a long time. If you're becoming inefficient, it's a sign that you need to either step away or to change things up; maybe aim for shorter, more intense bursts of work (I have colleagues who love pomodoros, for example), rather than long slogs in front of the computer.
Whatever you do, yes, it's normal and don't beat yourself up over it. Just take care of yourself and keep getting up in the morning!